Bike in the car deck and can normally be locked to something, but better to take your panniers up, if only because you're not allowed back down during sailing if you forget something.
Also second getting a cabin if possible. I did Portsmouth - St Malo in a chair and it was shit.
Thanks for the reply, it has been very useful.
By the way, is the bike stored on a safe place on the ferry or should I take with me panniers and all the stuff?